Pyrithiones are well-known antimicrobial additives that are useful in a myriad of applications. Sodium pyrithione (also called the sodium salt of 1-hydroxy-2-pyridinethione, sodium pyridine-2-thiol-N-oxide, or 2-pyridinethiol-1-oxide, Na salt) is one pyrithione salt having excellent antimicrobial properties, and is typically employed as a biocide and preservative in functional fluids, such as metalworking fluids, lubricants, cosmetics and toiletries. Sodium pyrithione is a well-known commercial product commonly made by reacting 2-chloropyridine-N-oxide with NaSH and NaOH, as disclosed, for example, in U.S. Pat. No. 3,159,640.
Likewise, zinc pyrithione also known as zinc pyridine-2-thiol-N-oxide or bis 1-hydroxy-2(H) pyridinethionato!-zinc! is an excellent antimicrobial additive. Zinc pyrithione may be made by reacting 1-hydroxy-2-pyridinethione or a soluble salt thereof with a zinc salt (e.g., ZnSO.sub.4) to form a zinc pyrithione precipitate, as disclosed, for example in U.S. Pat. No. 2,809,971. Zinc pyrithione has been employed as a broad-spectrum anti-microbial agent and preservative in metalworking fluids, plastics, paints, adhesives and cosmetics. Its principal uses are as an anti-dandruff agent in hair products, as a preservative in various cosmetics, and as an antifoulant in marine paints. The commercial use of zinc pyrithione in paints, adhesives, caulks and sealants is growing.
In the presence of ferric ion, sodium or zinc pyrithione-containing compositions tend to turn blue, even when the ferric ion is present in mere trace amounts, much less when higher amounts of the ferric ion is present. Likewise, in the presence of ultraviolet radiation, which is a component of natural, outdoor light, photodegradation of the pyrithione moiety may occur. This blue or yellow discoloration is undesirable for aesthetic reasons, as well as for functional reasons relating to unwanted color formulation.
Since the aesthetics of paints, adhesives, caulks and sealants normally require certain desirable colors after drying to form a dry coating on a substrate, and since the formulators of such products go to great lengths to achieve specific color effects, any ingredient which causes the formulation to vary much from a desired white or color may make the colorant formulators' task very difficult. For example, when formulating water-based paints, paint bases (i.e., the partially formulated paint before pigment addition), adhesives, caulks and sealants, any unwanted color in an additive can adversely affect the color of the formulated product, Such discoloration typically adversely affects the desired product color, producing an off-color product.
Likewise, yellowing of the dry coating film on a substrate, by virtue of ultraviolet ("uv") exposure as a component of natural outdoor light, adversely affects the paint in two respects. First, the yellowing of the paint is an aesthetic problem since the paint changes color upon uv exposure. Second, photodegradation of pyrithione attributable to uv exposure diminishes the amount of pyrithione available to protect the paint film.
In the past, various solutions to the blue discoloration problem have been proposed. By way of illustration, U.S. Pat. Nos. 4,957,658 and 4,818,436 disclose solutions to the above-discussed discoloration problem attributable to the presence of ferric ion plus pyrithione, in paints and functional fluids (e.g., metalworking fluids) respectively, by adding to the paint or functional fluid an alkali metal or alkaline earth metal salt of 1-hydroxyethane-1,1-diphosphonic acid. Although the '658 and '436 patents propose good solutions to the discoloration, these solutions are not always as cost effective or permanent as might be desired.
As another illustration, U.S. Pat. No. 4,161,526 discloses a white to cream yellow pyrithione, pyrithione salt or dipyrithione for application to skin or hair containing from about 0.01 percent to about 1 percent of the zinc salt of an organic carboxylic or inorganic acid, zinc hydroxide or zinc oxide, or a mixture thereof. The composition of the '526 patent is said to be effective in preventing or removing discoloration caused by formation of a colored pyrithione, pyrithione salt, or dipyrithione contaminant (said to be iron pyrithione) in the composition. The '526 patent does not teach a solution to the discoloration problem in compositions unrelated to skin or hair care, and not containing iron pyrithione.
In addition to the discoloration problem, unwanted ions, such as ferric ion, can cause a functional problem with respect to the antimicrobial performance of pyrithione. It is believed by the present inventors that this performance problem results from the fact that the pyrithione tends to form a blue precipitate in the presence of ferric ion, and precipitation of pyrithione reduces the amount of available pyrithione throughout the composition, thereby diminishing the biocidal protection thereof.
Moreover, the present inventors have found that the presence of a large amount of zinc oxide in a pyrithione-containing paint can surpress the short-term (or initial stage) of antimicrobial efficacy imparted by the pyrithione to the paint when dried to form a dry film on a substrate. This diminished short term efficacy adversely affects the performance of the paint, particularly in regard to resistance of the paint film to mildew growth.
New solutions to the yellow, blue and green discoloration problem in various pyrithione-containing aqueous coating compositions, particularly paints, adhesives, caulks and sealants would be highly desired. Preferred solutions would includes those that enable pyrithione to be utilized in coating compositions containing iron or copper, is without encountering any resulting discoloration of the composition, and that is cheaper, longer lasting, and/or uses lower levels of additives than required by the above-discussed prior art. A particularly preferred solution, namely one providing improved short-term and long-term antimicrobial resistance of the coating composition and resulting coating against microbial attack, would be highly desired by the paint, adhesives, caulks and sealants manufacturing community. The present invention provides one such solution.